Diagnostic approach in dogs with increased ALP activity (Proceedings)

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An increase in serum alkaline phosphatase (ALP) activity is a common laboratory finding in dogs. It is typically used as a diagnostic marker for cholestatic liver disease. However, increased ALP activity has a high sensitivity (86%) but poor specificity (49%) for canine liver disease. This is due to the large number of non-hepatic diseases and drugs that may induce production of this enzyme. Alkaline phosphatases comprise a heterogeneous group of enzymes, widely distributed in mammalian cells, for which the exact physiologic function is unknown. ALP has been isolated from the kidney, liver, bone, placenta, and intestine of dogs. Dogs also produce a unique isoform, in response to increased endogenous or exogenous corticosteroids. Although numerous isoforms have been identified, only three have long enough half-lives (approximately 70 hours) to be detected in canine serum. These include the liver isoform (L-ALP), bone isoform (B-ALP), and corticosteroid isoform (C-ALP). Isoforms from the intestine, kidney, and placenta are not detected in the serum because of the short half-life (minutes). The total ALP is routinely measured. The C-ALP isoform, but not the L-ALP or B-ALP isoform can be specifically measured in some diagnostic laboratories.

The proportion of isoforms in the serum of normal dogs varies with age. In dogs less than one year of age, the B-ALP predominates, because of increased osteoblastic activity that occurs with bone growth. In dogs older than one year, L-ALP predominates. C-ALP comprises 10-30% of total ALP in normal dogs. As dogs age, the proportion of B-ALP progressively decreases to approximately 25%, and the proportion of C-ALP increases to about 30%. The liver isoform is associated with bile canaliculi and sinusoidal membranes. Increased L-ALP occurs with hepatobiliary disease, both from increased production of the enzyme and elution from hepatocyte and biliary epithelial membranes. The largest increases occur with cholestasis. Phenobarbital is the best characterized example of drug-induction of L-ALP. Glucocorticoids also induce L-ALP as well as C-ALP (discussed below).

C-ALP is synthesized de novo by the liver in dogs after exposure to endogenous or exogenous glucocorticoids. C-ALP cannot be used as a diagnostic test for hyperadrenocorticism (or recent glucocorticoid administration), because it is increased in many other situations beyond excess glucocorticoids. Most dogs with hyperadrenocorticism have increased C-ALP, which accounts for 70-100% of the total ALP; however, specificity for hyperadrenocorticism is low (18%). Consequently, the clinical usefulness of C-ALP in the evaluation of dogs suspected to have hyperadrenocorticism, is that the absence of C-ALP makes hyperadrenocorticism unlikely. Other causes of increased C-ALP include phenobarbital therapy, diabetes mellitus, primary liver diseases, and acute or chronic stress or illness that is associated with increases in endogenous glucocorticoids. Progesterones are also thought to induce C-ALP production in dogs.

The effects of corticosteroid administration on serum ALP have been studied. Increased ALP is due to enzyme induction, and does not reflect hepatic dysfunction. Dogs treated experimentally with immunosuppressive doses of prednisone (4.4 mg/kg/day IM) for 10 days had an increased total ALP by 3 days with progressive increases over the 10-day period. Initially, the isoform was primarily L-ALP. C-ALP was first detected at 7 days, but by day 10 was still only 6% of the total ALP. This probably reflects delayed expression of the steroid ALP gene expression in liver tissues. With exogenous corticosteroid administration, the percentage of C-ALP increases over time. In the clinical setting, dogs treated with corticosteroids (dose and duration not defined) had 60-100% of the total ALP attributed to C-ALP. There appears to be substantial individual variation. Topical ophthalmic and otic preparations that contain glucocorticoids can also have significant systemic absorption, induce ALP activity, and cause clinical features of hypercortisolemia. When corticosteroids are discontinued, increases in ALP activity will persist for variable periods of time, depending on the dose, duration, and type of corticosteroid. Three weeks duration is suggested for short-acting glucocorticoids (prednisolone) and 5 weeks for longer-acting corticosteroids. Higher (immunosuppressive doses) will also prolong the duration. Most dogs with iatrogenic hyperadrenocorticism (caused by a variety of preparations and dosages), had a return of ALP to baseline by 6 weeks. However, dogs receiving prednisone at 4.4 mg/kg/day IM for 14 days, ALP remained increased at 8 times baseline at 6 weeks after stopping therapy.

B-ALP is attached to the cellular membrane of osteoblasts. Increased serum levels are seen with bone growth in young animals, osteosarcoma, fracture healing, and nutritional bone disease. Increases in B-ALP in dogs with osteosarcoma are usually mild (less than 4x the upper limit of normal) and are associated with a poor prognosis. Increases are found in tumors with increased osteoblastic activity (osteoblastic osteosarcoma). Benign familial hyperphosphatasemia, a rare condition described in Siberian Huskies, is associated with increased B-ALP.

Causes of increased total ALP activity can be classified as: 1). Age (young dogs) and breed-related considerations (Siberian Huskies with familial hyperphosphatasemia; Scottish Terriers); 2) Drug-induced (corticosteroids, phenobarbital, others); 3) Cushing's Disease; 4) Primary hepatobiliary disease (intra- and extrahepatic cholestatic disorders, including vacuolar hepatopathy, cholangitis, hepatitis, neoplasia, nodular hyperplasia); 5) Systemic disorders causing reactive hepatopathy or physiologic stress associated with acute or chronic illness (neoplasia, infection/inflammation; pancreatitis; GI disease; endocrine disorders such as diabetes mellitus, hypothyroidism) and 6) Bone-related disorders (neoplasia, nutritional osteopathy, hyperparathyroidism)

Evaluation for the cause of increased serum ALP activity should include a complete history, physical examination, and laboratory tests consisting of a CBC, biochemical profile, and urinalysis. Review of this initial database of information will usually provide insights into many of the causes of increased ALP activity, and will often suggest the next diagnostic step. For example, dogs less than 15 months of age with mild increases in ALP activity (< 2 times upper limit of normal), are often due to increased B-ALP, from normal bone growth. Concurrent mild increases in calcium and phosphorus are often present. These findings are consistent with a normal age-related variation, and do not require further investigation. A complete drug history, (especially glucocorticoids and phenobarbital) is essential, including topical ocular or dermatologic preparations that may contain corticosteroids. Over-the-counter drugs, and herbal and dietary supplements should also be considered, since ALP induction is possible or these products may cause hepatotoxicity. If a medication related increase is suspected, discontinuing the medication and monitoring the ALP should be considered. Return of ALP activity to normal indirectly supports a drug-related cause.

Cushing's disease is often suspected based on the typical clinical findings (PU/PD, polyphagia, panting, abdominal enlargement, truncal alopecia, thin skin), and biochemical features (increased ALP/C-ALP activity, mild increases in ALT activity, normal total bilirubin and other parameters reflecting hepatic function; dilute urine). However, the medication history should be carefully reviewed, since findings are similar with exogenous or endogenous corticosteroids. With hepatobiliary disease, laboratory features may include increased ALP activity accompanied by increased ALT activity, and evidence of cholestasis (hyperbilirubinemia, hypercholesterolemia, bilirubinuria) or hepatic synthetic dysfunction (hypoabuminemia, decreased BUN, hypoglycemia).

Clinical findings in dogs with systemic or non-hepatic disorders causing reactive hepatopathy (or stress-related increases in ALP activity) are referable to the primary disease. Liver enzyme elevations (ALT - 2 times the upper limit of normal; ALP - 3 to 4 fold increases) are common; thus mimicking primary hepatic disease. However, tests that reflect liver function, (including serum bile acids), are usually normal. Further diagnostic testing is directed toward the primary disease.

Common conditions that are associated with only an increase in serum ALP activity include hyperadrenocorticism, idiopathic vacuolar hepatopathy, hepatic neoplasia, hepatic nodular hyperplasia, drug induction, and breed-related disorders. Scottish terriers are reported to have higher ALP activity than other breeds. A recent study suggests that hyperphosphatasemia is predominantly due to C-ALP, and is likely due to hyperadrenocorticism. However, clinical signs of hyperadrenocorticism were not present and dogs were apparently healthy.

The next step for more advanced evaluation often includes an abdominal ultrasound with an FNA of the liver for cytology, and serum bile acids. Findings of adrenal enlargement, hyperechoic enlarged liver, and hepatic vacuolar changes on cytology, would lead to screening test for Cushing's disease (Low dose dexamethasone suppression test or ACTH stimulation test). Vacuolar hepatopathy is a common feature in dogs with excess glucocorticoids, either from glucocorticoid administration, Cushing's disease, or increased levels of endogenous corticosteroids with chronic illness or stress. Benign nodular hyperplasia is a common finding in asymptomatic older dogs. Hyper- or hypoechoic nodules may be seen on ultrasound examination, or the liver may appear normal if nodules are isoechoic to hepatic parenchyma. FNA often reveals vacuolar change. If biochemical and ultrasonographic features support primary hepatobiliary disease, and bile acids are abnormal, a liver biopsy is usually warranted. Isolated increases in ALP activity that remain unexplained after complete diagnostic evaluation, have the potential to be benign, in contrast to isolated increases in ALT activity.

Suggested Reading: Zimmerman KL et al. JAVMA 237:178-186, 2010; Fernandez NJ et al. Vet Clin Path 36:223-233, 2007; Gary AT et al. Kirk's Current Veterinary Therapy XIV, 549-553, 2010; Sepesy LM et al. JAVMA 229:246-52, 2006